One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique of restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using a cardiac defibrillator.
To determine if defibrillation is required, defibrillators usually rely on an interpretation of an electrocardiograph (ECG) signal that is displayed on an ECG monitor or plotted on a strip of paper by a chart recorder. In such systems, the ECG signal is displayed as a waveform normally containing the P and T waves, as well as the QRS peaks associated with ventricular contraction. These waveforms are interpreted to determine the presence of ventricular fibrillation, ventricular tachycardia, asystole (the absence of contractions of the heart), or other abnormal heartbeat patterns.
One of the problems related to ECG monitoring is the typical situation where the ECG electrodes and certain tissues have become charged due to currents drawn through them during a pace pulse or defibrillation pulse. Once charged, the electrodes discharge over a time interval that may last up to several hundred milliseconds, depending upon the chemistry of the electrodes. As they discharge, the voltage across them is not DC, but rather an approximately exponentially decaying waveform, that is substantially larger than the normal range of ECG signals. Such voltages drive the amplifiers of the ECG monitoring circuit into saturation. Thus, a common problem is how to return as quickly as possible to normal ECG monitoring following the application of a defibrillation or pacing pulse which charges the electrodes and drives the amplifiers into saturation.
One prior art method for dealing with this problem is illustrated in U.S. Pat. No. 5,609,611 to Bolz et al., which discloses a pacemaker system with a porous electrode and residual charge or after-potential reduction. In addition, U.S. Pat. No. 4,811,738 to Economides et al. discloses a cardiac pacemaker with fast stored charge reduction. Both of these patents deal with the problem of trying to measure the electrical "response signal" of the heart (such as an ECG signal) during the period following a stimulation pulse. As stated in the patents, during each stimulation pulse, electrical charge is stored on the electrodes and in the polarization of the stimulated tissue that can interfere with normal heart measurements. Both disclosures describe techniques of providing short charges of opposite polarity following a stimulation pulse, so as to counteract the stored charge and quickly restore normal heart measurements. While these methods address the stored charge issue, they have disadvantages in that they require extra energy and time to implement the countercharges, and require additional countercharge circuitry.
Another prior art method is shown in U.S. Pat. No. 5,447,518 to Pless, which discloses a method and apparatus for phase-related cardiac defibrillation. As shown in FIG. 1 of Pless, the output from a first amplifier is coupled to a DC baseline restoring circuit including a resistor, a second amplifier, and a capacitor. The baseline restoring circuit has a variable time constant controlled by a microprocessor through a switch. When the switch is closed, current from the first amplifier is shunted through the resistor, thus providing a faster time constant. In addition, the second amplifier has a variable DC set point, which is also under the control of the microprocessor. In general, the sensing circuit generates a defibrillation output signal in response to an ECG signal of a certain level. After a defibrillation output signal is generated, the microprocessor sets the DC baseline restoring circuit to the rapid time constant by closing the switch. After a few milliseconds, the output of the amplifier is back to the same potential as it was just prior to the application of the defibrillation output signal. The microprocessor then opens the switch to return the baseline restoring circuit to the slower time constant. While this circuit addresses some of the problems raised regarding monitoring an ECG waveform following the application of a defibrillation pulse, it also has certain disadvantages. For example, in some systems, the method of instantly switching the baseline restoring circuit between rapid and slower time constants in the presence of decaying offsets tends to produce erroneous QRS detect marks (related to the QRS waveforms that indicate ventricular contraction) that could be interpreted as indicating a properly functioning heart, when in fact the heart is experiencing ventricular fibrillation. This could lead an operator to refrain from applying a defibrillation pulse when one is needed.
Accordingly, a method and apparatus are needed for returning to normal ECG monitoring as quickly and seamlessly as possible following delivery of defibrillation therapy. The method and apparatus should not require the output of additional energy or the presence of a significant amount of additional circuitry. In addition, the method and apparatus should return to normal monitoring without producing erroneous QRS detect marks. As explained in the following, the present invention provides a method and apparatus that meet these criteria and solve other problems in the prior art.